If it wasn't obvious already, this adds 1000 power to an Epheria Sailboat. The sailboat itself has one million power points, and you use 1000 per five seconds... so this adds five seconds for four million silver.

Even a +10 Black Breeze Sail only nets you an extra 30 seconds of sailing. Needless to say, the only reason to get these is for the appearance; don't waste your money upgrading.